Comments on Missing a solution: Are A and B always equal if A-B=0
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Missing a solution: Are A and B always equal if A-B=0
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I just came across this problem on an online learning app:
4x(5−x)−12(5−x)+100=100
I tried to solve it by subtracting 100 from each side then inferring if something minus something else equals zero then those two things must be equal.
4x(5−x)=12(5−x)
Divide both sides by (5-x) and x must be 3.
But brilliant had a different way of solving it that revealed there are two possible values for x.
Where did I make a mistake in my approach to solving the problem?
Is it incorrect to assume that if a minus b equals 0 then a must equal b?
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Dividing by 5-x is valid only if x is not 5. But x=5 is another solution to the original equation.
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